1. Field of the Invention
This invention relates to a stepping motor and a driving method thereof and, more particularly, relates to a composite type three-phase stepping motor having no reduction gear box for rotating directly a transfer drum of a reproducing machine or the like.
2. Description of the Prior Art
A transfer drum of a reproducing machine or the like is driven by a brushless motor. The rotary speed of the motor is reduced by using a reduction gear box from 1500 rpm to 30 or 60 rpm, for example.
FIG. 1 shows a vertically sectional front view of a conventional inner rotor three-phase hybrid type stepping motor for driving the transfer drum or the like. FIG. 2 shows a cross section taken along lines 2--2 of FIG. 1.
In FIGS. 1 and 2, reference numeral 11 denotes a stator, 11c a stator winding, 12 a rotor, 12G a permanent magnet, 13 a rotor shaft, and 14a.sub.1 -14a.sub.6 stator magnetic poles. Symboles A.sub.1 and A.sub.2 denote one pair of stator magnetic poles for an A phase, B.sub.1 and B.sub.2 denote another pair of magnetic poles for a B phase, and C.sub.1 and C.sub.2 denote the other pair of magnetic poles for a C phase.
FIG. 3 shows a driving circuit D or D' for the three-phase hybrid type stepping motor, wherein the winding 11C.sub.1 of A phase, a winding 11C.sub.2 of B phase, and a winding 11C.sub.3 of C phase are connected to (+) and (-) terminals of an electric power source (not shown) through six transistors Tr.sub.1 -Tr.sub.6.
To each of the transistors Tr.sub.1 -Tr.sub.6, a base current of a predetermined value is supplied from the electric power source.
One of the transistors Tr.sub.1, Tr.sub.3, Tr.sub.5 at the upper side of said six transistors and one of the transistors Tr.sub.2, Tr.sub.4, Tr.sub.5 at the lower side of the six transistors, for example, are switched ON to supply currents to two-phase windings in the three-phase windings, respectively.
Specifically, if the transistors Tr.sub.1 and Tr.sub.4 are switched ON, the winding 11C.sub.1 of A phase becomes (+) potential and the winding 11C.sub.2 of B phase becomes (-) potential, and the entire sequence of energization of windings can be shown in FIGS. 4A to 4D.
FIGS. 4A to 4D show time charts indicating the operation of the driving circuit shown in FIG. 3. FIG. 4A show trigger pulses to be supplied to the transistors Tr.sub.1 -Tr.sub.6 respectively. FIGS. 4B, 4C and 4D show currents to be supplied to the windings 11C.sub.1 of A phase, 11C.sub.2 of B phase and 11C.sub.3 of C phase, respectively.
The above-mentioned conventional stepping motor has many problems and defects.
1 In the inner rotor three-phase hybrid type stepping motor as shown in FIGS. 1 and 2, if the outer diameter of the motor is about 60 mm, the resolution is less than 600 (step angle is 0.6.degree.).
In order to obtain a resolution of more than 600, the number of the rotor pole tooth (Nr) must be increased to more than 100, so that the width of the pole tooth becomes smaller than the thickness of the magnetic iron plate forming the rotor core.
This means that the rotor cannot be formed by pressing.
When the transfer drum of the reproducing machine etc. is driven by such stepping motor having the resolution of less than 600, the fluctuation in rotation becomes large, so that the stepping motor must be rotated at a high speed and the speed must be reduced.
Further, due to the backlash etc. of the reduction member, it is difficult to enhance the color image quality of the copy formed by the reproducing machine and to prevent the color doubling from being occurred.
In order to solve such problems, it has hitherto been proposed a direct drive system wherein the rotation of the motor is directly transmitted to the transfer drum without reducing the speed.
Such direct drive system has been disclosed in the Japanese Patent Laid-Open NOS.208668/1990 and 235573/1992.
The Japanese Patent Laid-Open NO.208668/1990 discloses a method of rotating in a predetermined direction an electrophotographic photosensitive drum (transfer drum) by using a flat motor as a prime mover, the rotary speed of the electrophotographic photosensitive drum being controlled by a servo mechanism.
The Japanese Patent Laid-Open NO.235573/1992 discloses a driving method of a transfer drum of a LBP wherein a brushless motor having an encoder is used as a prime mover, and the rotary speed of the brushless motor is controlled by an output of the encoder.
Another method of controlling the rotary speed of a stepping motor by an output signal of an encoder mounted on the stepping motor has been proposed.
However, either method must be carried out by using means for controlling the rotary speed of the motor, which is complicated in construction and high in cost.
On the other hand, a direct drive system simple in construction wherein a transfer drum is driven directly by a stepping motor having no encoder has been proposed.
In this system, no controlling means is required, however, the step angle of such stepping motor is 0.6.degree..
It is necessary to limit the fluctuation of the rotary speed of the transfer drum for a plain paper copier(PPC) or (LBP) to less than 0.2% (peak to peak) when the transfer drum is rotated at 60 rpm in order to maintain a good image quality.